The present invention relates to a method of preparing clarithromycin using erythromycin A N-oxide; and to a novel intermediate prepared in said method.
Clarithromycin, 6-O-methylerythromycin A, is a semisynthetic macrolide antibiotic which exhibits a wide range of antibacterial activity against gram positive bacterium, some gram negative bacterium, anaerobic bacterium, Mycoplasma, Chlamidia and Helicobacter pylori (see U.S. Pat. No. 4,331,803).
Clarithromycin can be prepared by methylating the 6-hydroxy group of erythromycin A. However, selective methylation of the 6-hydroxy group is difficult to achieve because erythromycin A has four reactive-hydroxy groups besides 6-hydroxy group as well as a dimethyl amino group which can undergo quaternarization during the methylation reaction.
To deal with such a problem, various methods have been developed for the preparation of clarithromycin.
For example, there has been disclosed a general method of preparing clarithromycin using an erythromycin A 9-oxime derivative, as an intermediate (see EP Patent Nos. 0,158,467; 0,195,960; 0,260,938; and 0,272,110 and International Publication Nos. WO 97/36912 and WO 97/36913). Although this method gives a relatively high yield, it is hampered by a low productivity due to the fact that the process requires a number of reaction steps including the steps of oximization, protecting the oxime group, removing the oxime protection group, and deoximization.
Another general method for preparing clarithromycin has been reported in EP Patent Nos. 0,147,062 and 0,177,696. The method disclosed in EP patent No. 0,147,062 comprises the steps of: protecting the 2-hydroxy and amino groups of erythromycin A with benzyloxycarbonyl groups; methylating the 6-hydroxy group; removing the two benzyloxycarbonyl protecting groups; and methylating the amino group to obtain clarithromycin. However, this method suffers from the problem of a low yield of 13.1 to 18.5% and requires the use of excess benzyloxycarbonyl chloride, besides the difficulty of applying the column chromatographic separation of the product in mass production.
Accordingly, there has existed a need to develop an improved method for preparing clarithromycin.
It is, therefore, an object of the present invention to provide a method of preparing clarithromycin.
It is another object of the present invention to provide a novel intermediate prepared in said method.
In accordance with one aspect of the present invention, there is provided a method of preparing clarithromycin of formula (I) consisting essentially of the steps of:
(a) preparing erythromycin A N-oxide of formula (II) from erythromycin A;
(b) reacting erythromycin A N-oxide of formula (II) with a methylating agent to obtain 6-O-methylerythromycin A N-oxide of formula (III); and
(c) treating 6-O-methylerythromycin A N-oxide obtained in step (b) with a reducing agent to obtain clarithromycin: 
In accordance with another aspect of the present invention, there is provided 6-O-methylerythromycin A N-oxide of formula (III).
The compound of formula (I) may be prepared starting from erythromycin A N-oxide as follows:
Step (a)
Erythromycin A N-oxide of formula (II) is produced in a yield of greater than 98% from erythromycin A according to a well-known method [E. H. Flynn et al., J. Am. Chem. Soc., 76, 3121(1954) and P. H. Jones and E. K. Rowly, J. Org. Chem., 33, 665 (1968)].
Step (b)
6O-methylerythromycin A N-oxide of formula (III) is produced by reacting erythromycin A N-oxide of formula (II) with a methylating agent in an organic solvent in the presence of a base.
Exemplary methylating agents which may be suitably used in the present invention are methyl bromide, methyl iodide, dimethyl sulfate, methyl p-toluenesulfonate, methyl methanesulfonate and a mixture thereof. The methylating agent may be used in an amount ranging from 1 to 3 equivalents based on the amount of erythromycin A N-oxide of formula (II).
Exemplary solvents that may be used in the above methylating reaction include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, N,N,Nxe2x80x2, Nxe2x80x2, Nxe2x80x3,Nxe2x80x3-hexamethylphosphoramide, tetrahydrofurane, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methoxyethylether, 2-ethoxyethylether, 1,2-bis(2-methoxyethoxy)ethane, tetraethylene glycol dimethylether, acetone, acetonitrile and a mixture thereof.
Further, a base selected from the group consisting of alkali metal hydrides, hydroxides and alkoxides, e.g., sodium hydride, potassium hydride, sodium hydroxide, potassium hydroxide and potassium t-butoxide, may be suitably used in this process. The base may be used in an amount ranging from 0.9 to 2 equivalents based on the amount of erythromycin A N-oxide of formula (II). The methylating reaction may be carried out at a temperature ranging from xe2x88x9215 to 40xc2x0 C., preferably from 0xc2x0 C. to room temperature.
After the completion of the methylating reaction, water is added to the resulting mixture and the mixture is extracted with chloroform. The extract is concentrated and acetone is added to the residue, and then, stirred to precipitate by-products. The resulting mixture is filtered to remove the by-products and the filtrate is concentrated to obtain crude 6-O-methylerythromycin A N-oxide (yield: 67 to 73% and purity: 50 to 57%).
The crude 6-O-methylerythromycin A N-oxide obtained above may be used as is in step (b), or purified by recrystallization from ethylacetate to obtain refined 6-O-methylerythromycin A N-oxide (purity of 83% to 88% and yield of 40 to 44%), which may be further purified by recrystallization from chloroform to obtain 6-O-methylerythromycin A N-oxide crystals having a purity of greater than 95%.
Step (c)
Clarithromycin of formula (I) is prepared by reacting 6-O-methylerythromycin A N-oxide obtained in step (b) with reducing agent in an organic solvent to remove a N-oxide.
Exemplary reducing agents which may be suitably used in the present invention are hydrogen in the presence of a hydrogenation catalyst such as palladium, Raney-nickel or platinum oxide(PtO2); a nickel-aluminum alloy(Nixe2x80x94Al alloy) combined with potassium hydroxide; metallic zinc in the presence of formic acid or acetic acid; sodium hydrogen telluride(NaTeH); samarium iodide(SmI2); stannous chloride(SnCl2); hexabutylditin(Bu3SnSnBu3); cyclohexene and osmium tetroxide(OsO4); and ferrous sulfate, and preferred reducing agents in practicing the present invention are stannous chloride(SnCl2), hexabutylditin(Bu3SnSnBu3), a nickel-aluminum alloy combined with potassium hydroxide, and hydrogen in presence of a Raney-nickel or platinum oxide(PtO2) catalyst.
In case stannous chloride is used as a reducing agent, solvents such as methanol, ethanol, isopropanol, ethyl acetate, acetonitrile, acetone, tetrahydrofuran, 1,2-dimethoxyethane, dichloromethane, chloroform and a mixture thereof may be used. The amount of stannous chloride used ranges from 1 to 3 molar equivalents based on the amount of 6-O-methylerythromycin A N-oxide of formula (III) and the reaction may be carried out at a temperature ranging from 0xc2x0 C. to the boiling point of the solvent used, preferably from 10 to 45xc2x0 C. After the completion of the reaction, clarithromycin may be isolated by (1) neutralizing the reaction mixture with a base such as triethylamine; adding water thereto; and extracting the mixture with an organic solvent; or (2) adding water to the reaction mixture; neutralizing the resulting mixture with a base such as sodium bicarbonate, sodium carbonate, sodium hydroxide solution, and aqueous ammonia; and extracting the mixture with an organic solvent.
In case hexabutylditin is used as a reducing agent, solvents such as ethyl acetate, acetonitrile, acetone, tetrahydrofuran, 1,2-dimethoxyethane and a mixture may be used. Hexabutylditin used in an amount ranges from 1 to 3 molar equivalents based on the amount of 6-O-methylerythromycin A N-oxide of formula (III) and the reaction may be carried out at a temperature ranging from room temperature to the boiling point of the solvent used.
When a Nixe2x80x94Al alloy is used together with potassium hydroxide as a reducing agent, a mixture of water and a lower alcohol, e.g., methanol and ethanol is employed as a solvent. The amounts of the Nixe2x80x94Al alloy and potassium hydroxide used are in the range of 0.5 to 3 g and 2 to 20 moles, respectively, based on a mole of 6-O-methylerythromycin A N-oxide of formula (III). The reaction may be carried out at a temperature ranging from room temperature to the boiling point of the solvent used.
When catalytic hydrogenation is carried out as a means of conducting the reaction of step (c), a lower alcohol or a mixture of water and a lower alcohol may be used as a solvent and, a hydrogenation catalyst such as Raney-nickel at a temperature ranging from room temperature to the boiling point of the solvent under a hydrogen atmosphere.
The reaction of step (c) may also be effected using an inorganic reducing agent such as sodium bisulfite(NaHSO3), sodium sulfite(Na2SO3), sodium thiosulfate(Na2S2O3), sodium hydrosulfite(Na2S2O4), sodium pyrosulfate(Na2S2O5), sodium thionate(Na2S2O6), potassium bisulfite(KHSO3), potassium thiosulfate(K2S2O3) and potassium pyrosulfate(K2S2O5). The reaction may be carried out in mixture of water and a lower alcohol such as methanol, ethanol and isopropanol at a temperature ranging from 0xc2x0 C. to the boiling point of the solvent used. The reducing agent may be used in an amount ranging from 1 to 20 equivalents, preferably from 1 to 4 equivalents based on the amount of 6-O-methylerythromycin A N-oxide of formula (III).
In carrying out the reaction of step (c) using sulfur oxides as a reducing agent, 3xe2x80x2-N-desmethyl-6-O-methylerythromycin A of formula (IV) may be formed as a by-product besides clarithromycin of formula (I). In such a case, the by-products may be converted to clarithromycin by methylating the secondary amine group of the by-product using formic acid and formaldehyde, in accordance with a known method [see Eschweiler and Clarke, Org. React., 5, 290(1945)]. 
The following Reference Example and Examples are intended to further illustrate the present invention without limiting its scope; and the experimental methods used in the present invention can be practiced in accordance with the Reference Example and Examples given herein below, unless otherwise stated.
Further, percentages given below for solid in solid mixture, liquid in liquid, and solid in liquid are on the bases of wt/wt, vol/vol and wt/vol, respectively, unless specifically indicated otherwise.